“Brake-by-wire” braking systems are becoming ever more widespread in the motor vehicle industry. Such braking systems often comprise a pedal decoupling unit, which is inserted ahead of a brake master cylinder, and, as a result, a brake pedal actuation by the driver in the “brake-by-wire” operating mode does not lead to direct actuation of the brake master cylinder by the driver. Instead, the brake master cylinder is actuated by an electrically controllable pressure supply device, i.e. is “externally” actuated, in the “brake-by-wire” operating mode. In order to give the driver a pleasant pedal feel in the “brake-by-wire” operating mode, braking systems generally comprise a brake pedal feel simulation device. In these braking systems, the brake can be actuated on the basis of electronic signals, even without the active intervention of the vehicle driver. These electronic signals can be output by an electronic stability program ESC or an adaptive cruise control system ACC, for example.
International Patent Application WO 2008/025797 A1 discloses a braking system of the above-referenced kind. In order to be able to dispense with expensive temporary storage of hydraulic actuating energy, which is unfavorable in terms of energy, the proposal is that the pressure medium required for electric control of the pressure fed into an intermediate space used for actuating the brake master cylinder should be held ready in the unpressurized form in the pressure supply device and subjected to a higher pressure when required. For this purpose, the pressure supply device is, for example, formed by a cylinder-piston assembly, the piston of which can be actuated by an electromechanical actuator. No method for controlling the braking system, in particular the pressure supply device, is described.
It is therefore the object of the present invention to provide a method for controlling an electrohydraulic “brake-by-wire” braking system having an electrically controllable pressure supply device comprising a cylinder-piston assembly, the piston of which can be actuated by an electromechanical actuator.
This object is achieved by a method in accordance with this invention.
It is advantageous if the method according to the invention is performed in a motor-vehicle braking system which can be activated either by the vehicle driver or independently of the vehicle driver in a “brake-by-wire” operating mode, preferably being operated in the “brake-by-wire” operating mode, and having the capacity for operation in at least one fallback operating mode in which only operation by the vehicle driver is possible.
An actuator position control operation is preferably performed when a mechanical end stop of the actuator is supposed to be detected.
It is likewise preferred that the pressure or actuator position control operation should be followed by an actuator speed control operation, in which the current actuator speed is adjusted to the target value for the actuator speed output by the pressure or actuator position control operation.
It is advantageous if the motor torque output by the motor torque feedforward function is taken into account in the actuator speed control operation.
According to a development of the invention, the pressure target value, advantageously the driver's required pressure target value, is formed from the sum of a first target pressure component and a second target pressure component.
The second target pressure component is preferably determined in accordance with the brake pedal actuation speed and a pedal speed threshold.
The pedal speed threshold is particularly preferably determined using a predetermined functional relationship from a pedal position/travel.
It is advantageous if the pedal speed threshold is selected in accordance with the vehicle speed. Thus, the value calculated for the pedal speed threshold using the functional relationship can additionally be multiplied by a function of the vehicle speed.
According to a preferred embodiment, a quotient of the brake pedal speed and the pedal speed threshold is calculated, and the second target pressure component is determined in accordance with the magnitude of the quotient, wherein the second target pressure component is calculated from the quotient and the first target pressure component, advantageously when the quotient is greater than one.
According to another preferred embodiment, a pressure gradient, in particular an expected pressure gradient, is determined, and a pressure control operation or an actuator position control operation or a combined pressure/actuator position control operation is performed on the pressure supply device in accordance with the magnitude of the pressure target value and/or the magnitude of the pressure gradient.
It is preferred if the pressure target value is used to determine a first component target value for the actuator rotational speed in an actuator position controller and is used to determine a second component target value for the actuator speed in a pressure controller, and if a target value for the actuator speed in a speed control operation on the pressure supply device is determined from the first and second component target values.
The target value for the actuator speed is preferably determined from the first and second component target values by weighted addition. As a particularly preferred option, the respective weighting factor is determined in accordance with the expected pressure gradient. As a very particularly preferred option, the weighting factors are determined from the pressure gradient using at least one predetermined function.
Exclusive pressure control of the pressure supply device is preferably performed if the pressure target value is greater than zero bar and the pressure gradient is less than a predetermined, positive, first value.
Exclusive actuator position control of the pressure supply device is preferably performed if the pressure target value is greater than zero bar and the pressure gradient is greater than a predetermined second value and, in particular, there is no brake control intervention.
It is advantageous if combined pressure/position control of the pressure supply device is performed if the pressure target value is greater than zero bar and the pressure gradient is greater than a predetermined first value and less than a predetermined second value and, in particular, there is no brake control intervention.